Figures: 2, 41, 46, 62, 71, 84, 92, 113, 133-134, 155-157, 185-186, 198-202, 206-208, 228
In addition to the Heptageniinae characteristics, the following combination of characteristics will identify the genus:
Larvae of Heptagenia are characterized by having gills that are inserted laterally, gill lamellae that are somewhat rounded (only rarely slender) with a marginal anal rib (Fig. 92), fibrils present on gills 1-7, a lyre shaped hypopharynx (Fig. 62), and no fan shaped robust setae on the tergum. The shape of the hypopharynx will separate larvae of Heptagenia from all other Heptageniinae except the southeast Asian genus Trichogenia. Heptagenia can be differentiated from Trichogenia by the absence of fan shaped robust setae on the terga, and Holarctic distribution.
Males of Heptagenia are characterized by having well-developed dorsolateral spines on the penes, no lateral spines on the base of the penes, and relatively slender titillators. The penes of Heptagenia are variable in shape (Figs. 155-157), but they are never 'L' shaped, as in Stenonema and Maccaffertium, or subquadrate with robust titillators, as in Macdunnoa. Males of Dacnogenia and some Heptagenia are similar and can only be separated on a species by species basis. In the Palearctic, where both genera occur, no Heptagenia will have the combination of medially contiguous penes lobes with a V shaped emargination apically, and a small brown dot near the midpoint of the femora. North American females cannot always be distinguished from those of Maccaffertium and Macdunnoa, except by the presence of knob-terminated coiled threads on the eggs (Fig. 113).
Holarctic (Fig. 228).